This invention pertains to the art of radiation imaging detection systems, and more particularly to clinical nuclear detection systems having analog data derandomizing circuitry for reducing data loss due to pulse pile-up.
In one method of diagnosing certain illnesses radioactive isotopes are administered to a patient under study. Particular isotopes are selected which concentrate in certain types of tissue, with the degree of concentration in the tissue being dependent on the type of tissue. For example, a greater portion of an administered quantity of Iodine 131 will collect or concentrate in the tissue of the thyroid gland than in surrounding tissue. The isotope emits an amount of gamma radiation proportional to its concentration, and the surrounding tissue absorbs a varying amount. Gamma radiation emitted from the tissue is detected and graphically presented as an image on a suitable readout device, such as a cathode ray device or a chart recorder. The image is a valuable aid in diagnosing the condition of the tissue under examination.
One well-known type of device developing an image of the distribution of a radioisotope is the scintillation camera. Scintillation cameras generally have a relatively large disc-shaped scintillation crystal which is positioned so that the crystal intercepts the gamma radiation emitted by the patient. The crystal scintillates upon absorbing gamma-ray energy and provides pulses of light energy. A thallium-activated sodium iodide crystal is conventionally employed as the scintillation crystal.
A plurality of phototubes are positioned near and optically coupled to the crystal so that a scintillation is normally detected by several of the phototubes. Each of the detecting phototubes develops an electrical signal having an amplitude proportional to the intensity of light received by it from a scintillation and the received intensity is a function of the brightness of scintillation and its distance from the phototube. The signals developed by the phototubes are amplified and applied to appropriate electronic computing circuitry which develops electrical data signals representative of the situs and the intensity of the scintillation. One such gamma radiation imaging camera system is disclosed in the above-referenced patent entitled SCINTILLATION DETECTOR INDICATING SYSTEM.
A major problem with previous radiation imaging detection system has been data loss due to the phenomena commonly referred to as pulse pile-up. Radiation emanating from a radioisotope intercepts the scintillation crystal generally at random time intervals. The probability of a pulse being generated in any time interval t is given by the equation P = 1-e.sup.-.sup.-.sup..delta.t/.sup..tau., where .tau. is the average period between pulses. Thus, several scintillation events may occur in the crystal in rapid succession, and then there may be a relatively long interval of time before a succeeding scintillation event occurs. When several scintillation events occur in rapid succession, the corresponding data signals are developed in rapid succession with a minimum of separation between the signals.
In the operation of previous analog imaging detection systems, if successive data signals are presented for display while a signal is being displayed, the successive data signals are irrestorably lost. Thus, data signals which are spaced in a sequence by a time interval less than the display or processing time, are aborted and forever lost.
Some prior art radiation imaging systems are digital systems which digitize the preamplified signals, as described in the above referred SYSTEM patent. An attempt to solve the pulse pile-up problem in the digital system is described in the above-referenced DATA DERANDOMIZER application. In this digital derandomizer, the incoming digital data is stored in a shift register until it can be accommodated by the display. This shift register method of derandomization has proven successful in digital systems.
Digital systems exhibit a relatively slow system operating rate compared to contemporary analog systems. For example, a typical minimum pulse separation of 3-5 microseconds may be required in digital derandomized systems. The radiation detection and front end data processor section of the system is common to both digital and analog systems, and is capable of generating data pulses separated by only one or two microseconds. Digitizing is time-consuming, and even derandomizing the digital signals accordingly will not provide the fastest operating system. Only analog systems have the capability to provide overall system operating speed commensurate with the radiation detection circuitry and the front end data processor section.
Although the described shift register method of derandomization has proven successful in digital systems, it is not suitable for analog systems. Analog systems require the stored data to retain their amplitudes in proportion to the intensity of the detected scintillation; whereas digital systems store the data as binary numbers. For this reason, analog systems require a method of derandomization which allows the retention of pulse height data.
The prior art has attempted to alleviate the pulse pile-up problem in nonclinical analog, gamma ray-scintillator systems used for spectral analysis of gamma radiation as opposed to the detection of and reconstruction of the situs of gamma radiation events of preselected energy values. In well-logging operations a scintillation detector and a fast neutron source are positioned within the earth, and the scintillation detector is pulsed by a clock coincidentally with pulsing of the neutron source. The neutron source produces pulses of gamma rays which pass through the geological structure and strike the detector to produce an array of data pulses. The pulses are transmitted over several miles of cable and tend to become distorted and to pile up by the time they reach the data processing system on the earth's is surface.
In an attempt to remedy this pile-up problem, the prior art has suggested a data processing system which is responsive to both the clock and the data pulses. The system is initially reset by each pulsing of the neutron source and is conditioned to receive a series of data pulses which would otherwise reach the surface with insufficient time separation for data processing. The pulses are selectively gated into a bank of parallel charge and hold storage circuits, and are sequentially discharged from the charge and hold circuits upon expiration of various preselected time periods.
This type of system which relies on an initial time reference signal is unacceptable for clinical nuclear scanning. In clinical nuclear scanning the data pulses occur more randomly and do not occur as a consequence of a prearranged event which may be utilized as a time reference signal for initializing the storage circuits.